The invention relates to a method for determining a fluid contact level in a hydrocarbon fluid bearing formation which surrounds and/or underlays an underground borehole.
In many situations one or more exploration wells are drilled into an oil and/or gas bearing formation such that the well does not reach the oil-water, the oil-gas and/or the gas-water interface in that formation.
It is known from U.S. Pat. No. 5,621,169 to predict the hydrocarbon/water contact level for oil and gas wells on the basis of measured data from well log and core analysis information and on basis of a worldwide correlation of permeability and porosity to a function of capillary pressure, without making actual capillary pressure measurements.
European patent application 586001 discloses a method for generating by way of experimental tests with core samples, the capillary pressure curve in a porous medium.
U.S. Pat. No. 4,903,207 discloses a method for determining reservoir bulk volume of hydrocarbons from reservoir porosity and distance to oil-water contact level which distance is determined from log data and capillary pressure analysis of core data.
U.S. Pat. No. 4,282,750 discloses a tool which measures in-situ the partial water pressure in an oil bearing reservoir whilst the partial oil pressure is measured using previously known formation sampling techniques which involve taking a core sample and determining the partial pressure and density of the crude oil present in the pores.
A disadvantage of the known methods is that they require complex and time consuming core sample analysis and correlation techniques.
The present invention aims to provide a method of determining the fluid contact level in hydrocarbon fluid bearing formation in a more simple, accurate and direct manner, without require time consuming core sampling and core sample analysis procedures.
In accordance with the invention there is provided a method for determining the depth (DL) of a fluid contact between a first fluid (F1) having a fluid density (xcfx81F1) and a second fluid (F2) having another fluid density (xcfx81F2), which fluids are present in the pores of an hydrocarbon fluid bearing formation surrounding or underlaying an underground borehole, the method comprising:
lowering a pressure probe assembly to a depth (DP) into the borehole and pressing a pair of pressure probes against the borehole wall, one of said pressure probes being adapted to measure solely the phase pressure (PF1) of the first fluid (F1) in the pores of the formation surrounding the borehole, the other pressure probe being adapted to measure solely the phase pressure (PF2) of the second fluid (F2) in the pores of the formation surrounding the borehole; and
determining the depth of said fluid interface (DL) on the basis of the following equation:             D      P        -          D      L        =                    P        F1            -              P        F2                    g      ⁡              (                              ρ            F1                    -                      ρ            F2                          )            
where g is the gravitational acceleration.
Suitably, the first fluid is water and the second fluid is a hydrocarbon fluid, such as crude oil or natural gas, and the method is used to determine the free water level in a hydrocarbon fluid bearing formation where said free water level is located in or below the bottom of the borehole.
Alternatively, the first fluid is crude oil and the second fluid is natural gas.
In case the densities of the first and second fluid are not known, or not accurately known, it is preferred that the probe assembly is initially lowered to a first depth (I) and subsequently to a second depth (II) in the well and the pressure probes are actuated to take pore pressure measurements at each of said depths and the measurements are used to determine and/or verify the fluid densities xcfx81F1 and xcfx81F2 of the first and second fluids, according to the well-known formula:       ρ    F    =                    P        2            -              P        1                    g      ⁡              (                              D            2                    -                      D            1                          )            
where P2 is the pressure for the fluid measured at depth D2, P1 is the pressure for the fluid measured at depth D1 and g is the gravitational acceleration constant.
It is generally preferred that the measurements are made using a probe assembly which comprises
a first pressure probe comprising a first pressure transducer which is mounted in a measuring chamber of which one side is permeable to the first fluid and impermeable to the second fluid, which side is pressed against the borehole wall during a predetermined period of time while the pressure transducer is actuated; and
a second pressure probe comprising a second pressure transducer which is mounted in a measuring chamber of which one side is permeable to the second fluid and impermeable to the first fluid, which side is pressed against the borehole wall during a predetermined period of time while the second pressure transducer is actuated.